1. Field of the Invention
The present invention relates to a membrane electrode assembly used in a fuel cell.
2. Description of the Related Art
An increased amount of information due to advancement of electronics in recent years has resulted in a demand for processing of the increased information with higher speed and quality. Hence, a power supply with high output density and high energy density, i.e., a power supply with longer continuous operating time is needed.
A demand for a small generator that does not require charging, i.e., a microgenerator that can be easily refueled, has been growing. Under these circumstances, importance of fuel cells is being reviewed.
A fuel cell is a generator including at least: a solid or liquid electrolyte, and two electrodes, i.e., an anode and a cathode, which induce desired electrochemical reactions, and directly converts chemical energy of fuel in the fuel cell into electric energy, with high efficiency.
Among such fuel cells, a fuel cell that uses a solid polymer electrolyte membrane as an electrolyte membrane, and hydrogen as fuel is referred to as a polymer electrolyte fuel cell (PEFC), while a fuel cell that uses a solid polymer electrolyte membrane as an electrolyte membrane, and methanol as fuel is referred to as a direct methanol fuel cell (DMFC). In particular, the DMFC using liquid fuel has attracted attention as being effective as a portable or handheld small power supply because of high volumetric energy density of the fuel.
It is desirable to make a DMFC generating system as small as possible when the DMFC generating system is used as a portable or handheld power supply. Thus, the output density of a membrane electrode assembly, which is a power generating element of the DMFC, needs to be made higher.
In the DMFC, liquid fuel is supplied to the anode for oxidizing the fuel and air is supplied to the cathode for reducing oxygen, and a part of the liquid fuel penetrates through the solid polymer electrolyte membrane and moves to the cathode. This causes the cathode to be immersed in the liquid fuel having penetrated, thereby blocking the supply of the air. As a result, the output density is reduced.
In order to improve the output density of the DMFC, it is necessary to improve diffusivity of air into the cathode and thereby supply as much oxygen as possible to a cathode catalyst. For example, Japanese Patent Application Publication Nos. Hei 8-88008 and Hei 8-162123 describe methods of improving diffusivity of air by designing the cathode so that the farther away from the solid polymer electrolyte membrane a portion of the cathode is, the fewer amount of the solid polymer electrolyte the portion contains.
In addition, in the DMFC, it is desirable that a membrane electrode assembly, which is the heart of power generation, have a long life. In the DMFC, as power generation continues, a solid polymer electrolyte membrane is damaged in the vicinities of the ends of each electrode, and thus a large amount of liquid fuel flows into the cathode, which significantly deteriorates cell performance.